This invention relates generally to the field of control of antilock braking for vehicles, and in particular, to a system and method for vehicle stability enhancement control during ABS operation.
Antilock braking systems (ABS) are becoming more common on vehicles, particularly automobiles and light trucks. When the driver applies the brakes, ABS monitors whether a tire is skidding or about to skid, and automatically releases the brakes for a short period, so the wheel is allowed to rotate, rather than skid. The ABS control system controls braking pressure in either an applied, hold, or release state, cycling from one to the other throughout the stop, depending on a wheel""s skid condition. Typically, skidding is monitored at each of a vehicle""s four wheels by measuring wheel speed and looking for a sudden deceleration, indicating the wheel is no longer rolling, but is stopping and skidding.
Vehicle stability enhancement (VSE) has also become more common on vehicles. VSE reduces vehicle spins and excessive understeer, reducing the difference between the driver""s requested direction and the actual vehicle direction. VSE uses various sensors to look at the difference between the driver""s requested direction and the yaw rate, and then manages individual wheel braking to bring the vehicle back toward the driver""s requested direction.
Although ABS and VSE both improve stopping and handling in certain conditions, they typically work independently of each other. If a driver is turning and ABS is active because of a skid, VSE will take control of the wheel braking and brake pressure modulation. This strategy is largely successful, but has the following disadvantages: large brake pedal movements during VSE control which are potentially alarming to the driver; difficulty in controlling brake pressure, and therefore the wheel, under heavy driver braking while in VSE; and difficulty in smoothly returning control to ABS from VSE, because ABS does not have control or receive wheel information while in VSE.
Accordingly, it would be desirable to have a system and method of vehicle stability enhancement control during ABS operation that overcomes the disadvantages described.
One aspect of the invention provides a method of vehicle stability enhancement control during ABS operation, comprising the steps of determining whether ABS mode is active; determining whether VSE is required; calculating a target wheel speed for each wheel if ABS mode is active and VSE is required; calculating an adjusted wheel slip for each wheel by subtracting the wheel speed for each wheel from the target wheel speed for each wheel and dividing the difference by the target wheel speed for each wheel; and determining an ABS control mode for each wheel using the adjusted wheel slip.
Another aspect of the invention provides the step of calculating the target wheel speed for each wheel, in which the step of calculating the target wheel speed for each wheel further comprises adding a VSE modification term for each wheel to the vehicle speed to calculate the target wheel speed.
The invention provides the foregoing and other features, and the advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention and do not limit the scope of the invention, which is defined by the appended claims and equivalents thereof.